49-CAMELOPARDALIS - AN UNCOMMON MAGNETIC STAR

We have discovered that the magnetic variable 49 Cam displays a remark able linear polarization, with periodic variations larger than diose o f other Ap stars. The time variation of the Stokes parameters has been established with good accuracy for the standard B band (Figs. 1 and 2 ) and additional measurements in the U and V bands already give some i nsight into the spectral variation of the polarization (Fig. 3). 49 Ca m is expected to be closer than about 50 pc and its polarization, like that of most neighbouring stars, suffers little contamination from th e interstellar polarization. The hypothetical presence of continuum po larization due to a circumstellar shell seems unlikely, given both the spectral dependence of the polarization and the negative result of ou r polarimetry bearing on several A-shell stars. Therefore, we think th at the polarization of 49 Cam has to be understood, as for the other A p stars, as the result of magnetic intensification in spectral lines. The interpretation of the polarization variation is much easier if one knows the magnetic phase derived from the classical longitudinal magn etic field measurements. Owing to new measurements obtained at the Uni versity of Western Ontario, we have improved the knowledge of the ephe meris of 49 Cam and we can propose a more accurate rotation period (4. 2866 -/+ 0.0005 days). The specific shape of the linear polarization d iagram makes it difficult to interpret our observations in the customa ry framework of the oblique rotating dipole. Modifications of this mod el are necessary: as a first step, we have introduced local weighting functions which can represent either a change of the actual field stru cture, or, simply, a change of the saturation of the spectral lines. I n particular, one can reproduce fairly well the variations of the pola rization of 49 Cam if the magnetic signal is not symmetrical with resp ect to the magnetic axis and has some peculiarities in the polar regio ns (Figs. 4 and 5). In fact, the problem is not to find a magnetic sol ution but rather that there are too many possible solutions. While we cannot claim that we have already found a firm model for 49 Cam, there are serious guidelines for progress of the interpretation. In any cas e, it is clear that this star is one of the most peculiar objects amon g the 40 Ap stars considered in our investigation.